A conventional resonant converter has an advantage of allowing for zero voltage switching (ZVS) of primary side switches, and is widely used in applications where high efficiency, high frequency and high power density are required. However, in applications working in a wide voltage range (wide input voltage or wide output voltage range), there are various problems for the resonant converter, such as a wide switching frequency range, difficulty in design for efficiency optimization, and large-sized magnetic components, which make it difficult for the conventional resonant converter to achieve good performance throughout the entire wide voltage range.
At present, more and more research concentrates on the resonant converter applicable in the wide voltage range. For most of the research work, the basis is to broaden the voltage range of the resonant converter by adding additional circuit. These methods have disadvantages such as high cost and complex topology. On the other hand, without modifying the circuit topology, the U.S. Pat. No. 9,490,704B2 proposes a control strategy with “variable frequency and delay time”, which can narrow the switching frequency range, and thus can be applied to applications in a wide voltage range.
However, since the operating frequency of a traditional resonant converter is higher in light-load condition, the turn-off loss is relatively high, which plays a dominant role in switching loss; while according to the control method proposed in the patent document U.S. Pat. No. 9,490,704B2, the primary side current is smaller than traditional resonant converter in light-load condition, it is harder to achieve ZVS of primary side switches, thus the turn-on loss increases and its light load efficiency is poorer.